This invention relates to optical systems, and, more particularly, to an optical cement having a low index of refraction.
Optical devices for transmitting, conducting, and receiving data are of great interest because of the potential for replacing many electrical circuits with optical circuits. The optical circuits are light in weight, secure, resistant to many types of radiation, and of small dimension. More information can be transmitted through an optical line than through an electrical line of comparable size and weight. Just as electrical circuits require various types of devices and methods for their fabrication, optical circuits also require methods for fabricating specialized optical devices. The present invention deals with a method having wide applicability in the fabrication of optical devices, particularly waveguides and related circuit elements.
In some integrated optical circuits, light is guided through the optical circuit by means of planar optical waveguides. (These optical waveguides in the form of flat glass laminates operate to transmit electromagnetic energy of wavelengths in or near the visible spectrum (i.e., light) and include an inner planar glass core of transparent material which acts as the waveguiding layer, sandwiched between two transparent layers such as glass having a lower refractive index than the core.) Due to this difference between the index of refraction of the core and the cladding, total internal reflection occurs, and the light entering one end of the thin planar waveguide is internally reflected along its length. According to the principle of total internal reflection, light entering the waveguide with the proper entry angle will be internally reflected at the interface between the core and the cladding, and will proceed down the length of the waveguide with multiple internal reflections from the cladding layers on the two sides of the core, without any loss of intensity regardless of the multiple reflections.
If the waveguide is long, there must be such total internal reflection for the waveguide to be operable, as even a small percentage reduction of light intensity on each reflection would result in insufficient intensity of the beam emerging from the waveguide. Consequently, the optical waveguide must be carefully constructed so as to avoid any loss or leakage of light from the waveguide.
The glass used to construct an optical waveguide is highly perfect, with a low density of imperfections that could scatter light in a direction whereby the light would not be totally internally reflected. Moreover, to achieve the proper numerical entry angles for total internal reflection within the waveguide, the indices of refraction of the glass of the core and the cladding should be close together, and typically differ by only about 2 percent. For a typical planar optical waveguide, the core is BaK2 glass having a refractive index of 1.540, and the cladding is K5 glass having a refractive index of 1.522.
The core and the cladding must be joined together carefully so as to avoid surface imperfections that would scatter the light, or would cause irregularities such that the light is directed at the surface at an improper angle at which total internal reflection would not occur. In one method known in the art, the pieces of glass are carefully lapped to a flat surface. They are next heated to their softening temperatures and fused together, and then cooled to ambient temperature. Because of the small difference in the coefficients of thermal expansion of the glasses used to make the core and the cladding, and the large temperature difference between the fusing temperature and ambient temperature, thermal strains arise at the interface upon cooling, resulting in a rippled surface pattern at the internal interfaces between the core and the cladding on either side. This rippling effect causes excessive attenuation of the light at each internal reflection, reducing the efficiency of the optical waveguide.
It would be desirable, therefore, to have a technique for bonding the core and the cladding together which does not require excessive heating and cooling during manufacture. With this objective in mind, various optical bonding techniques have been developed. (Conventional optical adhesives or cements have indices of refraction greater than the glass of the waveguide and are therefore inoperable, because light can become trapped within the layer of cement.) Additionally, some do not bond well to glass, resulting in reduced strength and leaving unbonded regions that scatter the reflected light. Polymeric materials that are otherwise candidates for bonding agents often are inert and do not bond well to glass. Other bonding techniques may require complex surface treatments of the glass and moderate heating that results in a minor, but noticeable, degree of rippling at the internal interfaces.
Thus, there exists a need for a method of bonding glass pieces together for use in critical optical applications such as waveguides. The method should be operable with no or minimal heating of the glass pieces during bonding, and must result in a highly perfect interface that does not scatter light upon internal reflection. The bonding agent at the interface must have an index of refraction sufficiency less than the index of refraction of the cladding that light is not trapped in the interface upon internal reflection. The present invention fulfills this need, and further provides related advantages.